On Saturday, an in Montana, injuring over 50 and killing three. The cause is of the accident is unclear and the National Transportation Safety Board has launched an investigation into why the train toppled over.

The 7/27 train was coming from Chicago, bound for Seattle. Normally, the Empire Builder takes travelers on an adventure through the Midwest and through western wilderness, at points following the Lewis and Clark trail. But at around 4 p.m. on Saturday afternoon, Empire Builder 7/27 with its two locomotives and ten cars derailed near the small town of Joplin, Montana.

Eight of its cars left the tracks with some tipping over onto their sides. The train 141 passengers and 17 crew onboard. Passengers from different cars describe a frightening situation. A passenger in one of the cars that didn’t derail, CNN , said that the derailment was like extreme turbulence in a plane:

Megan Vandervest, a passenger on a sleeper car on the train, had been taking a nap when the derailment jolted her awake.

“I would describe the experience as kind of extreme turbulence on an airplane but louder, and there was kind of a lot of smoke smell. The first thought I really had when I woke up was, ‘Oh my God we’re derailing,’” she told CNN.

Others, like a passenger noted in this NBC report, describe being thrown from one wall into another:

Authorities don’t yet have an explanation for what caused the derailment and a NTSB team has been dispatched to investigate. Images from the scene show that the derailment occurred near a switch where one track splits into two.

Director of Railroad Engineering and Safety Program at the University of Delaware, Allan Zarembski, says that human factors are the largest cause of train derailments, the New York Times. However, he says this time those factors don’t appear to be present and the cause could be that something broke.

It’s also possible the track may have bucked in high heat, from the New York Times:

Some passengers reported that the train ride felt bumpy for many miles, which might signal a problem with the train’s suspension system. But even if a train’s crew takes note of a problem like that, its source could be difficult to identify while the train is running between cities, Mr. Zarembski said.

If the turbulence was more sudden, Saturday’s heat could also be to blame, said Russell Quimby, a retired accident investigator for the National Transportation Safety Board.

Mr. Quimby said he suspects the train may have hit a section of the track that had buckled from overheating.

The track was last inspected on September 23.

The derailment has left some travelers stranded in St. Paul, Minnesota, KARE 11. Some of the stranded travelers got hotels while others returned to Chicago or finished traveling by plane. Westbound Empire Builder 7/27 service departing from Chicago from now to the 28th while eastbound Empire Builder trains will not be operating in the same timeframe.

NTSB Vice Chairman Bruce Landsberg to hold a media briefing soon and we’ll update when more information is available.